1. Field of the Invention
This invention relates to a process for preparing a rubber-containing, styrene-type resin from which molded products of sufficient strength can be obtained, even when the resin is molded by a low-shear molding technique.
2. Description of the Prior Art
Powder molding techniques are known in the art in which molded products are formed under low-pressure and low-shear conditions by melting a resin powder composition and forming it by any of a variety of known methods such as rotation molding, the rock-and-roll process, the Heisler process, the Engel process and the sinter process.
This powder molding technique is a rapidly developing field and possesses numerous economic advantages for molding large-size products. It is generally applicable to a large variety of thermoplastic compositions, and the plastic materials that can be molded by it are not restricted except by their ability to be formed into a powder or a liquid. Examples in recommended use include HI (high impact) polystyrene and ABS resin compositions as well as such thermoplastics as polyethylene, polyvinyl chloride, polystyrene and cellulose acetate.
Nevertheless, in general whenever HI polystyrene or ABS resin compositions are subjected, for example, to rotation molding to form large molded products or products of complicated configuration, there are often obtained defective products, characterized by the presence of particles that failed to become molten, unsatisfactory surface finish, incomplete casting into slits and voids of the mold, and the existence of numerous unwanted openings or cavities in the molded products. It is also disadvantageous that when the molding temperature is increased to improve the fluidity and the properties of casting into slits and voids or improving the surface finish, the molded products are severely colored, resembling the appearance of having been scorched.
Furthermore, even when molded under the most favorable conditions, for example by regulating the particle size of the resin composition and using carefully designed molds, the products made from HI polystyrene or ABS resin compositions tend to have unacceptably low intrinsic impact resistance, and are unacceptably brittle.
ABS resin compositions and HI polystyrene resin compositions are characterized by a complex two-phase structure in which rubber is dispersed in a hard, brittle matrix continuous phase. Unlike polyethylene and polyvinyl chloride compositions which are characteristically of a homogeneous structure, the ABS resin compositions and HI polystyrene resin compositions do not lend themselves to powder molding techniques.
Nearly all of ABS resins and HI polystyrenes are molded by the injection molding process or the extrusion molding process. In those processes, the resins are rendered molten, blended sufficiently under high shearing conditions, and then are charged in a metallic mold through a nozzle under a high pressure, or are extruded. The resulting molded products have excellent impact resistance characteristic of ABS resins and HI polystyrenes containing rubber component which is present for the purpose of imparting toughness to the hard, brittle resins. In contrast, according to the rotation molding process, the molded products are obtained with minimal blending effect and pressure, because the molding is carried out by rotating the resin-charged metallic mold at a relatively slow speed with heating.
ABS resin and HI polystyrene which have a characteristic two-phase structure containing a considerable amount of a rubber component and which have been prepared by a graft polymerization reaction have very complicated melt viscosity characteristics. It may be supposed, further, that in a powder molding process wherein very little shearing force or pressure is applied, control of the molding properties of the resin is difficult, and that the mechanical properties of the two-phase structure of the resulting product are changed, unlike the molded products obtained by the injection molding or extrusion molding process. In fact, in many cases, the impact resistance of powder-molded products obtained from general ABS resins or HI polystyrenes is far lower than that of injection-molded products and the reinforcing effect of the rubber component is not realized sufficiently.
Thus, unlike polyethylene or polyvinyl chloride, ABS resin and HI polystyrene are not satisfactory when employed in the powder molding process.
It is known that straight chain, saturated polyesters of relatively low molecular weights are used conventionally as modifying agents for plastic products such as by blending them with polyvinyl chloride. These polyesters, if incorporated in an analogous way, by blending with the resin, in ABS molded products, do not improve its mechanical properties, particularly the impact resistance, even though the fluidity of the resin composition is increased to some extent. Thus, when such a polyester was added to ABS resin particles as described in detail below, and the blended mixture subjected to the powder molding process, the moldability or impact resistance of the molded product was not improved at all, and the product was brittle.